1. Field of the Invention
The present invention relates to an optical fiber component for spot size transition and a method of making the same. In particular, the present invention relates to an optical fiber component for spot size transition formed by splicing optical fibers with different core diameters and a method of making the same.
2. Description of the Related Art
Optical fiber communication networks have been established rapidly in recent years. The optical fiber communication network is basically formed by splicing an outdoor optical fiber cable and an indoor apparatus or the like. In a situation in which demands for a communication network are increasing, high density packaging of optical fibers is inevitable. For example, in intra-machine wiring or the like, since the number of optical fibers inevitably increases, saving of space for containing the optical fibers and wiring of the optical fibers are matters of major concern.
In order to contain and wire a large number of optical fibers in a limited space, it is necessary to bend the optical fibers with a small radius of curvature. However, if the optical fibers are bent with a small radius of curvature, light easily leaks and the quality of the communication network as a whole is degraded.
Thus, in order to prevent light from easily leaking, even if the optical fibers are bent with a small radius of curvature, a so-called high Δ optical fiber has started to be used, in which a refractive index difference between a core and a clad, that is, a relative refractive index difference Δ, is larger than that of a single mode (SM) fiber which is the conventional optical fiber used in communication networks. The relative refractive index difference Δ of the high Δ optical fiber is 0.5 to 2.5%, whereas the refractive index difference Δ of the SM fiber is about 0.3%. If the relative refractive index difference is increased in this way, since the core diameter decreases, the spot size also decreases. Here, the spot size is a parameter indicating spread of the electromagnetic field distribution, that is, field distribution of a propagation mode in an optical waveguide, and is also referred to as mode field diameter.
However, such a high Δ optical fiber is also eventually required to be spliced with the ordinary optical fiber which constitutes the optical fiber cable. As a result, a large transition loss is caused because mismatching occurs in a splice due to not only a difference between core diameters, but also a difference between spot sizes. For example, when the SM fiber and a high Δ optical fiber with a spot size, which is about half of that of the ordinary optical fiber, are spliced in an abutting state using a connector or the like, a large transition loss of about 2 dB occurs due to the difference in spot sizes.
In order to eliminate such mismatching in a spliced portion of a SM fiber and a high Δ optical fiber, the following two techniques are known. In one technique, after fusion-splicing the SM fiber and the high Δ optical fiber, the high Δ optical fiber is heated to thereby thermally diffuse a dopant in the fibers to expand the core diameter such that an optimal spot size is obtained. The other technique involves heating the high Δ optical fiber to thereby thermally diffuse a dopant in the fiber and expand the core diameter such that an optimal spot size is obtained and, then, cutting the part of the expanded core diameter to fusion-splice the high Δ optical fiber with the SM fiber (e.g., see Japanese Patent No. 2618500).
In addition, there is also known a technique which involves cutting the expanded part and mounting the high Δ optical fiber to an optical connector such that a cut face thereof becomes an light incident and outgoing end face (e.g., see Japanese Patent No. 2619130).
The above-mentioned conventional techniques have problems as described below.
Japanese Patent No. 2618500 and Japanese Patent No. 2619130 describe a technique involving expanding the core diameter of the high Δ optical fiber and, then, cutting the expanded part to splice the high Δ optical fiber with the SM fiber. However, in the case where the location of the cut is decided such that an optimal spot size is obtained, since a transition loss is confirmed after splicing the high Δ optical fiber and the SM fiber, it is difficult at the time of cutting to judge whether the cuffing location is always at a portion where the optimal spot size is obtained. Thus, a highly accurate cutting technique and experience are required.
In addition, in the case in which a core diameter is expanded by heating an optical fiber, the expanded core diameter may fluctuate depending upon heating conditions, and it is impossible to cut a large number of optical fibers always at the identical position when core diameters of the optical fibers are expanded. Thus, it is difficult to steadily optimizes spot sizes of a range number of optical fibers.
Moreover, in the case in which the high Δ optical fiber is connected to the optical connector with the core expanded portion as the incident and outgoing end face, since an advanced technique is required for grinding the incident and outgoing end face in order to obtain an optimal spot size, it is difficult to increase working efficiency. Thus, process management becomes complicated.